The present invention relates to an object lens for an optical data read-out apparatus.
An aspheric surface lens having a large aperture ratio, suitable for use as the object lens of an optical data read-out apparatus such as a video disc player, has been disclosed in Japanese patent No. 57-76512. FIG. 1 shows the lens which is described in that patent. In FIG. 1, a laser light beam 3 emitted from a light source such as a laser tube (not shown in the drawings) is refracted by an object lens 1, to be transmitted through a substrate 2a of a recording disc 2, and thereby form a spot of light upon a recording surface 2b. Data read-out is performed by sensing variations in the resultant level of light which is reflected from or is transmitted through the recording surface 2b. The incident laser light beam 3 first falls upon a refracting aspheric surface S.sub.1 of object lens 1, which satisfies the following aspheric surface equation: ##STR1## in which Y denotes height measured from the optical axis of the lens, X denotes the distance, from a point whose height above the optical axis is Y, to a plane which tangentially contacts the apex of the first refracting surface S.sub.1, C.sub.1 is the curvature at the apex of S.sub.1, and K.sub.1, A.sub.4, A.sub.6 and A.sub.8 are aspheric surface coefficients.
The second refracting surface S.sub.2, which faces the recording disc 2, is formed such as to satisfy the following aspheric surface equation: ##STR2## where C.sub.2 is the curvature at the apex of the second refracting surface S.sub.2, and K.sub.2 is an aspheric surface coefficient of surface S.sub.2. The aspheric surface coefficients K.sub.1, A.sub.4, A.sub.6, A.sub.8 and K.sub.2 may for example take the following respective values: EQU K.sub.1 =-2.41688 (3) EQU A.sub.4 =0.62875.times.10.sup.-2 ( 4 ) EQU A.sub.6 =-0.21838.times.10.sup.-3 ( 5) EQU A.sub.8 =0.67164.times.10.sup.-5 ( 6) EQU K.sub.2 =-149.999 (7)
In addition, the radius of curvature r.sub.1 (=1/C.sub.1) of the first refracting surface S.sub.1 at the apex of that surface, the radius of curvature r.sub.2 (=1/C.sub.2) of the second refracting surface S.sub.2 at the apex of that surface, the distance between the apexes of the first and second refracting surfaces S.sub.1 and S.sub.2 (i.e. the thickness d of object lens 1), the distance between the second refracting surfaces S.sub.2 and the recording disc 2 (i.e. the working distance WD), and the index of refraction n.sub.1 of object lens 1, can, for example, take the following values, respectively: EQU r.sub.1 =3.3710 (mm) (8) EQU r.sub.2 =-14.768 (mm) (9) EQU d=4.0 (mm) (10) EQU WD=1.400 (mm) (11) EQU n.sub.1 =1.70214 (mm) (12)
The numerical aperture NA of such a lens has a value in the range of approximately 0.45 to 0.5, in order to provide a resolution of approximately 1,000 lines/mm, and correction for aberration is performed such as to hold residual aberration within the refraction boundary.
With such a prior art type of object lens, it is necessary for the focal distance f of the lens to satisfy the following conditions, in order to enable a sufficiently large value of working distance WD to be employed, while incorporating a sufficient degree of correction for aberration: ##EQU1##
In order to satisfy the above relationship (13), the focal distance f must be made relatively large. If the condition (13) is not satisfied, then it will not be possible to implement correction for astigmatic aberration. The necessity to satisfy condition (13) above makes it necessary for the object lens to be made large in size, if satisfactory correction for aberration is to be achieved and if a sufficiently large working distance WD is to be attained. Thus, it is difficult to make such a lens compact and light in weight.
Furthermore, with such a prior art type of object lens, both of the refracting surfaces S.sub.1 and S.sub.2 of the lens must be made to follow a relatively complex aspheric curvature, so that manufacture of such a lens is difficult.